4 5/8" Cutout size, Overall size 5" Know your boats most effective drive height, trim angle and propeller size to gain more speed and increase fuel efficiency.Inside the DataTachTM is a computer that receives GPS speed & RPM info and it calculates the slip of your boats propeller. It does this 5 times per second, to give you an accurate reading. The Livorsi DataTachTM Prop SlipTM Multi-Gauge is offered exclusively at Livorsi Marine Inc. Large easy to read LCD digits can be seen in direct sunlight. Fits into a Monster GPS cutout. Optional Recording Module. Available in a Mega or Race rim in 13 brilliant colors to tie into any existing dash. Available for in-dash or portable version. A few things that affect prop slip...Pitch of propeller, Propeller size, Condition of boat bottom, Condition of propeller, Engine mounting height, Weight distribution, Jack plate mounting and Trim angle.